The present invention relates to valves and in particular to instrument type needle valves having a rotating valve stem guided by a valve bonnet that is threaded into a valve body. The terms “valve stem”, “valve bonnet”, and “valve body” are sometimes referred to hereinafter as the “stem”, “bonnet” and “body” respectively. More particularly, the present invention relates to the metal-to-metal seal between the bonnet and body that prevents high-pressure fluid from escaping the valve.
One common method of securing the bonnet to the body is with the use of threads. The bonnet to body metal-to-metal seal may be placed before or after the threads. If the seal is placed before the threads, it is considered to be “below” the threads and internal to the valve. If the seal is placed after the threads, it is considered to be “above” the threads and on the exterior of the valve. It is preferred in the art to have a bonnet-to-body metal-to-metal seal below the threads so that process fluids running through the valve do not touch or “wet” the threads and compromise their integrity. Additionally, metal-to-metal seals placed below the threads have smaller sealing areas than seals placed above the threads. The smaller the area, the lower the force required to offset the pressure by fluids on the seal. The size of the thread determines the maximum amount of force that can be applied to a given area. Therefore, given a fixed thread size, seals placed below the threads can sustain higher pressures than seals placed above the threads. Alternately, given a fixed pressure, a reduction in thread size is possible with seals below the threads.
Many needle valves in the art have seals below the bonnet threads. However, as the physical size of the valve is reduced, the valve ports and fluid passageways become closer to the seal, and the reduced space often limits the possibilities for a seal below the threads. Fluid pressures dictate the amount of threads required for sealing, i.e., higher pressures require more threads. However, it may not be possible to shorten the distance between threads without reducing the operating pressure of the valve. An alternative approach is to reduce the ports' sizes to make room for the seal, but reduced port sizes (in order to have a seal below the threads) also undesirably reduce flow rates. Therefore, the body-to-bonnet seal of miniaturized needle valves often is placed above the threads, as in U.S. Pat. No. 6,820,857 B1, incorporated herein by reference in its entirety. It would be an advantage to provide a high pressure miniature needle valve that has a seal below the bonnet threads without reducing the port sizes and changing the flow characteristics of the valve.
Over time, even the best seal may leak from normal valve use. If a leak occurs between the body and bonnet, it is desirable in the art that by further tightening the bonnet the added compression on the seal will stop the leak. This is not easily accomplished with a metal-to-metal seal. The surface finishes on metal-to-metal seals determine how well they seal, and over-tightening the bonnet can destroy the surface. Further tightening of the bonnet may also exceed design stresses for both the threads and the sealing surfaces, and undesirable yielding or galling may occur as a result. Such yielding or galling is undesirable because mating surfaces of metal-to-metal seals require a high degree of accuracy in order for them to mate properly. In fact, parts are usually manufactured to desirably maintain close tolerances. If a leak occurs between the bonnet and stem, it is desirable in the art to be able to remove the bonnet from the body to access the stem seal for replacement, and thereafter to reassemble the valve without damage to the metal-to-metal seal. Therefore, a need exists for a reliable and repeatable bonnet-to-body metal-to-metal seal that allows for additional compression without damage to the seal.
Alternative sealing methods are used in the art in order to avoid the problems associated with metal-to-metal seals. One alternative sealing method is to use an elastomeric seal between the body and bonnet. However, valves that are under high pressures and elevated temperatures may not be able to employ elastomeric seals. It is known in the art that metal-to-metal seals far exceed the pressure and temperature limits of elastomers. Another alternative is to use a soft metal seal, such as brass, imposed between the body and bonnet that yields to the shape of the harder surfaces without destroying them. Because the physical properties of soft metal seals do not match the body and bonnet material, fluid compatibilities may restrict the use of the valve. If, for example, the body and bonnet are stainless steel, a corrosive fluid may attack the soft metal seal. The addition of any component including coatings that do not match the physical properties of the body and bonnet may raise concerns about fluid compatibilities and in service use.
Also, by adding a sealing component, two sets of leak paths exist (between the bonnet to sealing component and between the body to sealing component) instead of one (between the body and bonnet). In addition, the added sealing component and increased assembly times may raise the cost of the valve. In each case, the removal of the bonnet and replacement of the sealing component is required in order to repair the seal. By improving on the metal-to-metal seal, the added complexities and costs associated with alternative sealing methods can be avoided.
The disadvantages of the prior art are overcome by the present invention. Improved methods and apparatus are provided herein for effectively sealing a high-pressure fluid within a miniature needle valve.